Shortcut Without Confirmation Dialog
Preventing that this dialog appears seems not possible at the moment.



But you can define a 'custom operator' and 'create a shortcut' see: Create Shortcuts for Particle Edit Brushes? 

For daily work I've converted this into an Add-on, which creates the shortcut automatically:

 

Install the Add-on via File > User Preferences > Add-ons. Once it's enabled you can press Shift+P for Set Parent to Object and also Shift+Alt+P for Keep Transform operation in Object Mode:



parent_shortcuts.py

bl_info = {
    "name": "Parenting Shortcuts",
    "description": "Parenting Shortcuts",
    "author": "poor",
    "version": (0, 0, 1),
    "blender": (2, 75, 0),
    "location": "3D View",
    "category": "3D View"
}

import bpy

# operators
class SetParentToObject(bpy.types.Operator):
    """Tooltip"""
    bl_idname = "object.set_parent_object"
    bl_label = "Set Parent to Object"
    bl_options = {'REGISTER', 'UNDO'}

    def execute(self, context):
        bpy.ops.object.parent_set(type="OBJECT")
        return {'FINISHED'}

class SetParentKeepTransform(bpy.types.Operator):
    """Tooltip"""
    bl_idname = "object.set_parent_keep"
    bl_label = "Set Parent to Object (Keep Transform)"
    bl_options = {'REGISTER', 'UNDO'}

    def execute(self, context):
        bpy.ops.object.parent_set(type="OBJECT", keep_transform=True)
        return {'FINISHED'}


addon_keymaps = []

# register
def register():
    bpy.utils.register_class(SetParentToObject)
    bpy.utils.register_class(SetParentKeepTransform)

    # handle the keymap
    wm = bpy.context.window_manager
    kc = wm.keyconfigs.addon
    if kc:
        km = wm.keyconfigs.addon.keymaps.new(name='Object Mode', space_type='EMPTY')
        kmi = km.keymap_items.new(SetParentToObject.bl_idname, type='P', value='PRESS', shift=True)
        addon_keymaps.append((km, kmi))
        kmi = km.keymap_items.new(SetParentKeepTransform.bl_idname, type='P', value='PRESS', alt=True, shift=True)
        addon_keymaps.append((km, kmi))

# unregister
def unregister():

    for km, kmi in addon_keymaps:
        km.keymap_items.remove(kmi)
    addon_keymaps.clear()

    bpy.utils.unregister_class(SetParentToObject)
    bpy.utils.unregister_class(SetParentKeepTransform)


if __name__ == "__main__":
    register()